Thiol Reactivity toward Atomic Oxygen Generated during the Photodeoxygenation of Dibenzothiophene S-Oxide

Aromatic heterocyclic oxides, such as dibenzothiophene S-oxide (DBTO), have been suggested to release ground state atomic oxygen [O(P-3)] upon irradiation, and as such, they have been used to create a condensed phase reactivity profile for O(P-3). However, thiols, which are highly reactive with O(P-3) in the gas phase, were not previously investigated. An earlier study of O(P-3) with proteins in solution indicated a preference for thiols. A further investigation of the apparent thiophilicity provided the subject for this study. DBTO was employed as a putative O(P-3) precursor. However, the effective rate of O(P-3) formation was found to be dependent on reactant concentrations in certain cases. All reactants were found to increase the rate of deoxygenation to some extent, but in the presence of reactants containing an alcohol linked to a reactive functional group, deoxygenation occurred substantially more rapidly. The rate enhancement was quantified and attributed to the reaction of activated O atom within the solvent cage prior to escape into the bulk solution. Through competition experiments, the relative rate constants of O(P-3) with thiols and other functional groups were found. A small preference for primary thiols was observed over other thiols, sulfides, and alkenes. A much larger preference was observed for thiols, sulfides, and alkenes over aromatic groups. In summary, DBTO was successfully used as an O(P-3)-precursor, and the thiophilicity of O(P-3) was confirmed and quantified.